Generally, a transmission electron microscope is equipped with an astigmatism-correcting mechanism and a focusing mechanism to enable the operator to observe or photograph a magnified image of a specimen under the condition that the image is in focus and free of astigmatism. An electron microscope equipped with an automatic focusing device is disclosed in U.S. Pat. No. 4,680,469.
After an adjustment is made using the astigmatism-correcting mechanism or the focusing mechanism, an operator frequently directly observes an image to actually ascertain whether the image contains astigmatism or is in focus. Therefore, this determination requires experience and skill and is generally quite difficult to perform. In a transmission electron microscope, the image contrast deteriorates near the focal point, thus making it difficult to observe the image on the fluorescent screen. In recent years, a system has been adopted which allows the operator to observe an image picked up by a TV camera on a monitor. However, it is difficult to know the degree of deterioration in the image quality attributable to astigmatism unless the contrast and the brightness of the monitor are adequately adjusted. Hence, this system also necessitates skillfulness to check the conditions.